EVEN the Attorney-General can have a bad day in court. For Jeremy Mathews, the star witness in Western Court yesterday, it began with the claim that he was not qualified for the job, and got worse when his underpants - white - went on display. The underpants, along with Mr Mathews' suit, tie, shirt and vest, were submitted in plastic bags as exhibits in the case against Wong Kwai-nam, 28, who was alleged to have thrown a cup of ink over the Attorney-General in an attack outside the Legislative Council. Speaking under caution after his arrest, Mr Wong had said he had done it because Mr Mathews had ''betrayed justice'' and was not qualified to be Attorney-General. Wong, who dismissed his defence counsel because of a claimed failure to follow instructions, admitted throwing the ink, but he pleaded not guilty yesterday to common assault at Jackson Road, Central, on March 23. The question of Mr Mathews' worth as the territory's chief law officer went unconsidered. Magistrate Paul Kelly ruled that motive was irrelevant to a conviction, and found Wong guilty on the basis of his admission. Wong, cross-examining Mr Mathews earlier, asked whether he had felt ashamed as he was hit by the ink. Mr Mathews said he had not. Giving evidence, Mr Mathews said that as he had approached the Legco building for a meeting, a Chinese man had emerged and run towards him, carrying a cup inside a plastic bag. He said the man, Wong, had thrown the contents of the cup at his chest from an arm's length away. Wong had then circled to the right and attempted to hit him. He had held Wong's wrist and told him to stay calm, at which time a uniformed officer had arrived. Mr Mathews said he did not know Wong personally but understood he was a persistent petitioner on behalf of his elder brother, who was in jail for a serious offence. After being convicted, Wong reinstated his defence counsel, Bernard Chung, to speak in mitigation. Mr Chung said that by committing the offence, Wong had wanted to call the Attorney-General's attention to the unfair treatment of his family by the police and Legal Department. Private counsel A. R. Suffiad prosecuted on behalf of the Crown. Mr Kelly fined Wong $5,000 and ordered him to pay $2,000 compensation and $2,000 costs. However, it seems Mr Mathews is a good sport. A Legal Department spokesman said the Attorney-General would donate his $2,000 compensation to the Community Chest.